The 24i doesn't have a pilot light, but uses electric ignition, so if your boiler isn't firing there can only be a couple of causes...
The first thing to work out is whether or not the necessary spark is occuring. To do this, whip the front off and fire up the boiler while keeping a close look (but not too close!!!) on the two electrodes you will notice in the burner. You should observe a blue spark occuring between the electrodes, coupled with a clicking sound. If the spark is working you have problems with your gas flow, and you'll need to get a CORGI engineer in to look at this.
However, it is more likely that the spark is not occuring - and there are a couple of possible causes. Combi boilers are clever little fellas. They will carry out a few checks before they allow the system to fire up. For example, an air pressure switch will detect the movement of air in the flue. Therefore if the flue is blocked, or the fan is not working, the pressure switch will not be happy, and there will be no ignition spark. The first thing to check then is that the flue is not blocked, and the fan is working fine. Then check that the tubes connecting the flue to the pressure switch are not blocked. If this is all fine then fit a new pressure switch, as these can become dodgy - especially on the 24i. Its only about a tenner, so well worth a shot. Don't get the wrong one though, as this would be bad!! The part number you need for a 24i pressure switch is 87161424140.
Hope this helps, and good luck!!